(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to fabricate an image sensor cell
(2) Description of Prior Art
Image sensor cells are usually comprised of active image sensing elements, such as photodiodes, in addition to adjacent transistor structures, such as transfer gate transistors, and reset transistors. These transistor structures, as well as additional devices, used for the control and signal circuits, in the peripheral regions of the image sensor cell, are comprised with complimentary metal oxide semiconductor, (CMOS), devices. Therefore to reduce process cost and complexity, the image sensor cell has also been fabricated using CMOS process sequences. However the quality of the photodiode element, of the image sensor cell, can be degraded if subjected to traditional CMOS process sequences. For example the insulator spacer, formed on the sides of the reset transistor structure, can damage a semiconductor surface, such as an N type element, of the photodiode, resulting in image sensor characteristics, such as low signal to noise, (S/N), ratios, as well as high dark current generation.
This invention will describe a process sequence for forming insulator spacers on the sides of gate structures, after formation of the N type element, of the photodiode, however with the N type element, protected from the spacer etch procedure, by an overlying insulator layer, the same insulator layer used to create the insulator spacers. This novel sequence results in higher S/N ratios, as well as lower dark current propagation, than counterparts in which the surface of the photodiode was subjected to damaging process sequences, such as the spacer etch procedure. Prior art, such as Merrill in U.S. Pat. No. 5,789,774, as well as Merrill, in U.S. Pat. No. 5,841,176, describe fabrication sequences for an active pixel sensor, or for image censor cells, however these prior art do not show the novel process sequence, described in this present invention, in which an N type, photodiode element is protected for damaging spacer definition procedures.